1. Field of the Invention
The present invention relates to a heart pacemaker electrode of the type permanently attached to the heart for stimulating the heart by means of pulses supplied by the pacemaker, and for sensing heart activity.
2. Description of the Prior Art
Heart pacemaker electrodes are known which include at least one electrical conductor surrounded by an insulating sheath, which is inert to body fluid. The lead is provided with a conductive electrode head at its distal end, i.e., the heart-proximate end. Such heart pacemaker electrodes can be employed both for stimulating the myocardium and for detecting heart voltages. In order to maintain the necessary stimulation energy as low as possible, the current density should be as high as possible. This is achieved in the simplest manner by making the surface of the electrode head as small as is reasonably possible. When such electrodes are employed for sensing QRS potentials, they have a relatively high source impedance on the order of magnitude of 10-20 kohms, given a smooth electrode head surface. Because the input impedance of the pacemaker amplifiers generally lies on the same order of magnitude, these heart signals are greatly attenuated in the input stage. The sensing characteristics can be improved in the simplest manner by enlarging the conductive surface of the electrode head. The demands on the size of the electrode surface for simulating and sensing are thus competing, which has required conventional pacemaker electrodes to undertake an optimization as to electrode head surface area.
The problem of competing demands on the size of the electrode head surface, however, can also be resolved in other ways, for example, by making the surface of the electrode head micro-porous. As a result, current is concentrated during stimulation and thus the desired high current density is obtained, whereas the boundary surface from electrode head to electrolyte is large enough to obtain a low source impedance when sensing heart signals. Such electrode heads are known, for example, from a Siemens brochure describing endocardial electrodes having an electrode head of glassy carbon, dated March 1984 (A. 91003-M3372-L745-02-4J00).
A heart pacemaker electrode for simultaneous stimulation or sensing is also known from EP-A No. 0 032 356 or from U.S. Pat. No. 3,977,411. In this pacemaker electrode, two separate surfaces of respectively different sizes are separated from each other by an insulating layer. A larger of the surfaces functions as a sensing surface and is connected to the common conductor via a relatively high impedance resistor, and thus that surface does not substantially contribute to the current output during stimulation. The conductivity of the stimulation and sensing surfaces is the same, relative to the surface area.